1. Field of the Invention
This invention relates to semiconductor devices and more particularly to processing of titanium-tungsten (TiW) alloys during manufacturing of connectors for semiconductor devices.
2. Description of Related Art
In the manufacture of semiconductor devices electrical and mechanical interconnections employ titanium tungsten alloys as a part of the structure which interconnects layers composed of different materials. In patterning such interconnections it may be necessary to etch away unwanted material. When chemical etchants are used there is a problem of undercutting the structures where the titanium-tungsten alloys are exposed since that can seriously reduce the quality of the interconnection structures. The problem has been encountered in connection with Controlled Collapse Chip Connection (C4) solder balls where chemical etchants have been used to remove titanium-tungsten alloys. U.S. Pat. No. 5,462,638 of Datta et al. for "Selective Etching of TiW for C4 Fabrication", which is commonly assigned, describes a "chemical etchant . . . for removing thin films of titanium-tungsten alloy in microelectronic chip fabrication." Preferably the alloy removed is 10% Ti and 90% W, which is layered onto a substrate under chromium (Cr), and copper (Cu) seed layers for electrodeposition of C4 solder bumps which are to be formed into solder balls. The chemical etchant is an aqueous solution of 30% by weight of hydrogen peroxide H.sub.2 O.sub.2 and water to which are added EDTA and potassium sulfate (K.sub.2 SO.sub.4) which etches TiW rapidly at temperatures between 40.degree. C. and 60.degree. C.
U.S. Pat. No. 5,937,320 "Barrier Layer for Electroplated SnPb Eutectic Solder Joints" of Andricacos et al., which is commonly assigned, describes use of nickel (Ni) as an electroplating barrier layer for low temperature solder and refers to a first adhesion layer of titanium-tungsten (TiW) etched by a chemical process at 60.degree. C., as taught in Datta et al. U.S. Pat. No. 5,462,638.
Liu et al. U.S. Pat. No. 5,705,428 for "Method for Preventing Titanium Lifting During and After Metal Etching" creates several layers deposited upon a semiconductor substrate starting on a titanium (Ti) lower layer. The other layers include a titanium-tungsten (TiW) alloy layer formed on the Ti lower layer, followed by an aluminum-based (Al-based) layer with a trace of copper therein. Then a photoresist mask is formed over the Al-based layer. Thereafter, a series of Reactive Ion Etching (RIE) steps are used to "enable the transfer of the desired image to the underlying metal composite." The Al-based layer and the TiW layer are etched by RIE. Then the titanium lower layer is etched yielding titanium nitride sidewalls on the edges of the remaining portions of the titanium lower layer.
Alternatively the TiW layer is etched alone followed by stripping the photoresist with water in an oxygen ambient which forms titanium oxide sidewalls on the titanium lower layer. A third alternative is to etch the titanium in an N.sub.2 ambient to create titanium nitride sidewalls followed by stripping the photoresist with H.sub.2 O and O.sub.2 ambient. In summary, Liu et al. focus upon forming titanium nitride or oxide to prevent lifting of the lower Ti layer by including an appropriate gas (N.sub.2 or O.sub.2) in the RIE process used for defining metal lines. This RIE process etches to form nearly flat stacks of Al/TiW/Ti. Use of a RIE process for patterning Al/TiW/Ti layers is a well known prior art method which has been employed before the advent of Chemical Mechanical Processing (CMP) to define conductor lines. The focus of Liu et al. is the application of such an etching process that forms a TiN or TiO.sub.2 sidewall on the titanium lower layer to prevent lifting of the Ti layer.
A different kind of a process is described in Ozasa Yasuhiko Patent Abstracts of Japan: Application Number 01169491, with an application date of Jun. 29, 1989 published Feb. 14, 1991 for "Manufacture of Semiconductor Device" says, first coat a silicon nitride (Si.sub.3 N.sub.4) film with a TiW film as a bonding barrier. Next sputter a gold film onto the TiW lower film. A gold layer is plated onto the gold film by selective plating. Next coat the gold layer with an upper TiW film which is thicker than the original TiW film. Then pattern the gold layer, etch away the unprotected portion of the thicker upper TiW film, with a wet etching process which is shown to remove the TiW from the sidewalls of the gold layer and the exposed top surfaces of the gold film. Remove the photoresist. Next, the gold film and the upper TiW layer are etched away in two steps shown by the drawings using magnetron type RIE device to remove the gold film, first from layer exposing the top surface of the lower TiW film. Finally the TiW layers are etched away to complete the gold wiring where exposed, but the method of etching the last etching step is not defined in the abstract. The drawings show etching down to the silicon nitride (Si.sub.3 N.sub.4) film.